Two Step Poly Etch LDMOS Gate Formation

ABSTRACT

A method of making a transistor includes etching a first side of a gate, the gate including an oxide layer formed over a substrate and a conductive material formed over the oxide layer, the etching removing a first portion of the conductive material, implanting an impurity region into the substrate such that the impurity region is self-aligned, and etching a second side of the gate to remove a second portion of the conductive material.

TECHNICAL FIELD

The present disclosure relates generally to semiconductor devices, andmore particularly to a lateral double-diffused metal oxide semiconductor(LDMOS) device.

BACKGROUND

Voltage regulators, such as DC to DC converters, are used to providestable voltage sources for electronic systems. Efficient DC to DCconverters are particularly needed for power management in low powerdevices, such as laptop notebooks and cellular phones. Switching voltageregulators (or simply “switching regulators”) are known to be anefficient type of DC to DC converter. A switching regulator generates anoutput voltage by converting an input DC voltage into a high frequencyvoltage, and filtering the high frequency input voltage to generate theoutput DC voltage. Specifically, the switching regulator includes aswitch for alternately coupling and decoupling an input DC voltagesource, such as a battery, to a load, such as an integrated circuit. Anoutput filter, typically including an inductor and a capacitor, iscoupled between the input voltage source and the load to filter theoutput of the switch and thus provide the output DC voltage. Acontroller, such as a pulse width modulator or a pulse frequencymodulator, controls the switch to maintain a substantially constantoutput DC voltage.

LDMOS (laterally diffused metal oxide semiconductor) transistors areused in switching regulators as a result of their specific on-resistanceand drain-to-source breakdown voltage.

SUMMARY

In one aspect, a transistor includes a source region including a firstimpurity region implanted into a substrate, a drain region including asecond impurity region implanted into the substrate, and a gateincluding an oxide layer formed over the substrate and a conductivematerial formed over the oxide layer, the oxide layer comprising a firstside and a second side, the first side formed over a portion of thefirst impurity region and the second side formed over a portion of thesecond impurity region, the first side having a thickness of less thanabout 100 Å, and the second side having a thickness equal to or greaterthan 125 Å.

Implementations may include one or of the following features. Thethickness of the second side may be at least five times the thickness ofthe first side. The thickness of the first side may be about 70 Å orless. The thickness of the first side may be about 35 Å or less. Thesource may include a third self-aligned impurity region. A maximumdoping concentration of the third impurity region may be between about1×10¹⁷ atoms/cm² and 1×10¹⁸ atoms/cm². A doping concentration of thethird impurity region at a surface adjacent to the oxide layer may beless than about 5×10¹⁷ atoms/cm². The doping concentration may be lessthan about 3×10¹⁷ atoms/cm². The third impurity region may be positionedin a current path of the transistor. The second side may have athickness of between approximately 120 Å and 800 Å, e.g., betweenapproximately 200 Å and 400 Å. The transistor may be a lateraldouble-diffused metal oxide semiconductor (LDMOS).

In another aspect, a transistor includes a source region including afirst impurity region implanted into a substrate, a drain regionincluding a second impurity region implanted into the substrate, and agate including an oxide layer formed over the substrate and a conductivematerial formed over the oxide layer, the oxide layer comprising a firstside and a second side, the first side formed over a portion of thefirst impurity region and the second side formed over a portion of thesecond impurity region, the first side having a thickness such that aturn-on voltage of the transistor is less than 0.6V.

Implementations may include one or of the following features. Theturn-on voltage of the transistor may be between 0.4 and 0.5V. Thesource may include a third self-aligned impurity region. A maximumdoping concentration of the third impurity region may be between about1×10¹⁷ atoms/cm² and 1×10¹⁸ atoms/cm². A doping concentration of thethird impurity region at a surface adjacent to the oxide layer may beless than about 5×10¹⁷ atoms/cm². The doping concentration may be lessthan about 3×10¹⁷ atoms/cm². The third impurity region may be positionedin a current path of the transistor. The transistor may be a lateraldouble-diffused metal oxide semiconductor (LDMOS).

In another aspect, a transistor includes a source region including afirst impurity region implanted into a substrate, a drain regionincluding a second impurity region implanted into the substrate, anintrinsic diode, and a gate including an oxide layer formed over thesubstrate and a conductive material formed over the oxide layer, theoxide layer comprising a first side and a second side, the first sideformed over a portion of the first impurity region and the second sideformed over a portion of the second impurity region, the first sidehaving a thickness such that a turn-on voltage of the transistor is lessthan a turn-on voltage of the intrinsic diode.

Implementations may include one or of the following features. Theturn-on voltage of the transistor is between 0.4 and 0.6V. The sourcemay include a third self-aligned impurity region. A maximum dopingconcentration of the third impurity region may be between about 1×10¹⁷atoms/cm² and 1×10¹⁸ atoms/cm². A doping concentration of the thirdimpurity region at a surface adjacent to the oxide layer may be lessthan about 5×10¹⁷ atoms/cm². The doping concentration may be less thanabout 3×10¹⁷ atoms/cm². The third impurity region may be positioned in acurrent path of the transistor. The transistor may be a lateraldouble-diffused metal oxide semiconductor (LDMOS).

In another aspect, a method of making a transistor includes applying aphotoresist over a gate, the gate including an oxide layer formed over asubstrate and a conductive material formed over the oxide layer, usingthe photoresist as a mask, etching the gate to remove a portion of theconductive material, and using the photoresist and conductive materialas a mask, implanting an impurity region into the substrate such thatthe impurity region is self-aligned to the gate.

Implementations may include one or of the following features. The oxidelayer may be formed such that a first side of the oxide layer is thinnerthan a second side of the oxide layer. The first side may have athickness of less than about 100 Å, and the second side may have athickness that it at least five times the thickness of the first side.The photoresist may be less than about 0.5 μm thick. Implanting animpurity region may include bombarding the substrate with atoms at anangle that is less than 90° from a main surface of the substrate.Implanting an impurity regions may continues until a dopingconcentration of the substrate is between about 1×10¹³ atoms/cm² and5×10¹⁸ atoms/cm².

In another aspect, a method of making a transistor includes etching afirst side of a gate, the gate including an oxide layer formed over asubstrate and a conductive material formed over the oxide layer, theetching removing a first portion of the conductive material, implantingan impurity region into the substrate such that the impurity region isself-aligned, and etching a second side of the gate to remove a secondportion of the conductive material.

Implementations may include one or of the following features. The oxidelayer may be formed such that a first side of the oxide layer is thinnerthan a second side of the oxide layer. The first side may have athickness of less than about 100 Å, and the second side may have athickness that it at least five times the thickness of the first side.Implanting an impurity region may include bombarding the substrate withatoms at an that is less than 90° from a main surface of the substrate.Implanting the impurity region may continue until a doping concentrationof the substrate is between about 1×10¹³ atoms/cm² and 5×10¹⁸ atoms/cm².

In another aspect, a method of making a transistor includes applying afirst photoresist over a gate, the gate including an oxide layer formedover a substrate and a conductive material formed over the oxide layer,using the first photoresist as a mask, etching a first side of the gateto remove a first portion of the conductive material, implanting animpurity region into the substrate such that the impurity region isself-aligned, applying a second photoresist over the gate, and using thesecond photoresist as a mask, etching a second side of the gate toremove a second portion of the conductive material.

In another aspect, a method of making a integrated circuit includesforming a plurality of LDMOS transistors on a substrate, each LDMOStransistor including a gate oxide layer comprising a first side closerto a source side of the LDMOS transistor and a second side closer to thedrain side of the LDMOS transistor, the first side having a thickness ofless than about 100 Å, and the second side having a thickness equal toor greater than 125 Å, and forming a plurality of CMOS transistor on thesubstrate, wherein each CMOS transistor includes a gate oxide layer, andwherein forming the gate oxide layer of the CMOS transistor occurssimultaneously with forming the first side of the gate oxide layer ofthe LDMOS transistor.

Implementations may include one or of the following features. The gateof oxide layer of the CMOS transistor may be formed with the samethickness as the first side of the gate oxide layer of the LDMOStransistor. The thickness of the second side may be at least five timesthe thickness of the first side. The thickness of the first side may beabout 70 Å or less, e.g., about 35 Å or less. Forming the plurality ofLDMOS transistors may include depositing an LDMOS gate conductor andforming the plurality of CMOS transistor may include depositing a CMOSgate conductor, and the LDMOS gate conductor and CMOS gate conductor maybe deposited simulataneously. The LDMOS gate conductor and CMOS gateconductor may be polysilicon. The gate oxide layer of the CMOS gate mayhave a substantially uniform thickness.

Certain implementations may have one or more of the followingadvantages. A transistor having a gate oxide that is less than about 40Å can make the turn-on voltage of the transistor be less than theturn-on voltage of the intrinsic diode. A transistor having a turn-onvoltage that is less than the turn-on voltage of the intrinsic diode canreduce the recovery time of the transistor. Reducing the recovery timecan increase the efficiency of the transistor, particularly at highswitching rates. Applying a photoresist over the gate before implantingthe p-body can give an added layer of protection to avoid unwantedimplantation of impurities in the substrate. Etching through thepolygate a first time before implanting the p-body and then etchingthrough the polygate a second time after implanting the p-body ensuresthat a p-body is implanted only on the source side of the transistor,rather than on both the source and the drain side.

The details of one or more embodiments are set forth in the accompanyingdrawings and the description below. Other features, aspects, andadvantages will become apparent from the description, the drawings, andthe claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a buck converter.

FIG. 2 is a simplified circuit diagram of a buck converter.

FIG. 3 is a graph demonstrating the deadtime of a traditional buckconverter.

FIG. 4 is a schematic of an LDMOS transistor.

FIGS. 5A and 5B are schematics of an exemplary impurity profile in atransistor as described herein.

FIG. 6 is a graph of doping vs. distance from the surface for atransistor having a thin gate oxide thickness of 35 Å, 70 Å, and 125 Å,respectively.

FIGS. 7A and 7B are a chart, and corresponding graph, showing thecharacteristics of a transistor having a thin gate oxide thickness of 35Å.

FIGS. 8A and 8B are a chart, and corresponding graph, showing thecharacteristics of a transistor having a thin gate oxide thickness of 70Å.

FIGS. 9A and 9B are a chart, and corresponding graph, showing thecharacteristics of a transistor having a thin gate oxide thickness of125 Å.

FIGS. 10A-10K show an exemplary process of forming a transistor.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

When a transistor is used in synchronous rectification in a switchingregulator, efficiency losses occur as a result of reverse recovery ofthe intrinsic body diode charge up during deadtime, as well as diodeconduction during deadtime. By having a transistor in which the turn-onvoltage of the transistor is less than the turn-on voltage of theintrinsic diode, the efficiency of the switching regulator can beenhanced by reducing both mechanisms of loss associated with parasiticdiodes. The lower threshold voltage prevents minority carrier storage.Moreover, the conduction losses during deadtime will decrease by theratio of threshold voltage to diode turn-on voltage.

Referring to FIG. 1, a switching regulator 10 is coupled to a first highDC input voltage source 12, such as a battery, by an input terminal 20.The switching regulator 10 is also coupled to a load 14, such as anintegrated circuit, by an output terminal 24. The switching regulator 10serves as a DC-to-DC converter between the input terminal 20 and theoutput terminal 24. The switching regulator 10 includes a switchingcircuit 16 which serves as a power switch for alternately coupling anddecoupling the input terminal 20 to an intermediate terminal 22. Theswitching circuit 16 includes a rectifier, such as a switch or diode,coupling the intermediate terminal 22 to ground. Specifically, theswitching circuit 16 may include a first transistor 40, called ahigh-side transistor, having a source connected to the input terminal 20and a drain connected to the intermediate terminal 22 and a secondtransistor 42, called a low-side transistor, or synchronous transistor,having a source connected to ground and a drain connected to theintermediate terminal 22.

In one implementation, the first transistor 40 can be a Positive-ChannelMetal Oxide Semiconductor (PMOS) transistor, and the second transistor42 can be a Negative-Channel Metal Oxide Semiconductor (NMOS)transistor. In another implementation, the first transistor 40 and thesecond transistor 42 can both be NMOS transistors. In anotherimplementation, the first transistor 40 can be a PMOS, NMOS, or aLateral Double-diffused Metal Oxide Semiconductor (LDMOS), and thesecond transistor 42 can be an LDMOS.

The intermediate terminal 22 is coupled to the output terminal 24 by anoutput filter 26. The output filter 26 converts the rectangular waveformof the intermediate voltage at the intermediate terminal 22 into asubstantially DC output voltage at the output terminal 24. Specifically,in a buck-converter topology, the output filter 26 includes an inductor44 connected between the intermediate terminal 22 and the outputterminal 24 and a capacitor 46 connected in parallel with the load 14.During a high-side conduction period, the first transistor is closed,and the source 12 supplies energy to the load 14 and the inductor 44 viathe first transistor 40. On the other hand, during a low-side conductionperiod, the second transistor 42 is closed, and current flows throughthe second transistor 42 as energy is supplied by the inductor 44. Theresulting output voltage V_(out) is a substantially DC voltage.

The switching regulator also includes a controller 18, a high-sidedriver 80 and a low-side driver 82 for controlling the operation of theswitching circuit 16. A first control line 30 connects the high-sidetransistor 40 to the high-side driver 80, and a second control line 32connects the low-side transistor 42 to the low-side driver 82. Thehigh-side and low-side drivers are connected to the controller 18 bycontrol lines 84 and 86, respectively. The controller 18 causes theswitching circuit 16 to alternate between high-side and low-sideconduction periods so as to generate an intermediate voltage Vint at theintermediate terminal 22 that has a rectangular waveform. The controller16 can also include a feedback circuit (not shown), which measures theoutput voltage and the current passing through the output terminal.Although the controller 18 is typically a pulse width modulator, theinvention is also applicable to other modulation schemes, such as pulsefrequency modulation.

A simplified circuit diagram of a buck converter 200 is shown in FIG. 2.The buck converter 200 includes a high-side transistor 40, a low-sidetransistor 42, and an inductor 206. Each transistor has a correspondingintrinsic body diode, 212 and 214, respectively. A voltage V_(in), forexample 12V, is applied to the high-side transistor 40, and when thehigh-side transistor 40 is on, current will flow through the transistor40 and the inductor 206. In contrast, when the low-side transistor 42 ison, the inductor 206 will pull current from ground. Under normaloperation of the buck circuit 200, the regulator will switch betweenturning the high-side transistor 40 and the low-side transistor 42 on sothat the output of the filter 26 produces the desired voltage V_(out)(V_(out) somewhere between 0V and V_(in)).

To improve efficiency of the buck converter 200, it is desirable to havethe high-side transistor 40 on while the low-side transistor 42 is off,and vice versa. However, some downtime is required between the switchingin order to avoid having both transistors 40, 42 on and at same time,which can cause shoot-through and result in significant efficiencylosses and damage to the transistors. Thus, there is a short period, theintrinsic deadtime t_(d), between each high-side conduction and low-sideconduction period in which both transistors are open.

When both transistors 40, 42 are off, current through the inductor 206will not instantly drop to zero. The voltage across the inductor isdetermined by Equation 1:

V=L(di/dt),  (Equation 1)

where V is the voltage, L is the inductance, and i is the current in theinductor. As the inductor current decreases, the voltage at the inputend, i.e. near V_(in), of the inductor is forced to be negative. Whenthis voltage reaches approximately −0.7 V, the low-side body diode 214reaches its threshold voltage and begins conducting current into theinductor. As a result, in a traditional buck converter, the current willtravel through the diode 214.

When the current flows through the low-side diode, a number of losses inefficiency can result. The most significant loss is associated withreverse recovery. The reverse recovery loss is the loss associated withtaking the forward conducting diode from forward to reverse bias.Reverse recovery occurs when the high-side transistor is switched on. Inthe period before the high-side transistor is switched on, the low-sidebody diode is forward biased with the inductor drawing current throughthe diode from ground. In this state, the PN junction of the low-sidediode conducts, the depletion region is narrowed to its minimum width,and a buildup of charge carriers is formed on each side of the diode'sPN junction. When the high-side transistor is switched on, the low-sidediode goes from being forward biased at 0.7 V to being negative biasedat −12 V. However, the low-side diode does not instantaneously switchoff because the same buildup of charge carriers that allowed conductionacross the PN junction during forward bias causes a transient charge tobe depleted in negative bias. The reverse recovery loss is thus acurrent that flows through the high-side transistor in order to restorethe equilibrium reverse-biased charge across the low-side's PN junction.

The total amount of the diode recovery loss depends upon the outputcurrent, the parasitic inductance, and the high side drive capability.As shown in FIG. 3, the total deadtime t includes both the intrinsicdeadtime t_(d) and the reverse recovery time L. The reverse recoverytime t_(s) can account for a large fraction, e.g. more than 25% of thedeadtime of the transistor.

FIG. 4 shows a schematic cross-sectional view of an LDMOS transistor 400that could be used, for example, as a transistor in a buck converter,e.g., as the low side transistor 42, as discussed above. The LDMOStransistor 400 can be fabricated on a high voltage n-type well 402implanted in a p type substrate 404. A high voltage n-well implant istypically a deep implant and is generally more lightly doped relative toa CMOS n-well. The LDMOS transistor 400 includes a drain region 406, asource region 410, and a gate 412 with a stepped gate oxide layer 424.The drain region 406 includes a doped n+ region 414 and an n dopedshallow drain 416. The source region includes an n-doped n+ region 418,a p-doped p+ region 420, and a p-doped p-body 422. The impurities thatprovide the n-well 402, the n-doped shallow drain 416, and the n+regions 414, 418 are a first type of doping material, for examplephosphorous. Both the n-doped shallow drain 416 and the n-well 402 havelower concentrations of impurities than the n+ regions 414, 418.Likewise, the impurities that provide the p+ region 420 and the p-body422 are a second opposite type of doping material, for example boron.The p-body 422 can be self-aligned with the gate 412. In other words,the source-side edge of the gate and the gate-side edge of the p-bodycan be substantially aligned (subject to implantation effects that canforce a portion of the p-body 422 below the gate). Alternatively, thep-body 422 need not be self-aligned with the gate 412.

The p-body can have a maximum doping concentration of, for example,1×10¹⁷ atoms/cm² to 1×10¹⁸ atoms/cm². Moreover, the doping concentrationat the top surface 422 a of the p-body can be less than about 5×10¹⁷atoms/cm², such as less than 3×10¹⁷ atoms/cm², for example 2×10¹⁷atoms/cm². Exemplary doping profile is shown in FIGS. 5A and 5B.

Referring back to FIG. 4, the oxide layer 424 is located underneath thegate 412 and includes two portions, thin portion 424 a and thick portion424 b. Thin portion 424 a can be closer to the source 410 than thickportion 424 b and can partially overlap the n+ region 418 and the p-body422. Thick portion 424 b can be closer to the drain 406 than thinportion 424 a and can partially overlap the n+ region 414 and theshallow drain 406. As shown in FIG. 4, thin portion 424 a can be thinnerthan thick portion 424 b. Thin portion 424 a can be less than 100 Åthick, such as less than 40 Å thick, for example 35 Å. In contrast,thick portion 424 b can be at least five times as thick as the thinportion 424 a, such as at least 10 times as thick, for example between200 Å and 400 Å thick.

For the LDMOS transistor 400, a high enough positive voltage on the gate412, called the turn-on voltage (V_(t)), will push the positive holes ofthe p-body 422 away from the gate 412 to form a depletion layer. Thiswill create a channel for electrons (n) (an “n-channel”) to flow betweenthe source 410 and the drain 406. Varying the voltage between the gate412 and the substrate 404 modulates the conductivity of the n-channeland makes it possible to control the current flow between drain andsource.

The thin portion 424 a of the oxide layer 424, in combination withhaving a p-body profile as described above, can affect the turn-onvoltage (V_(t)) of the gate and transistor. As the thin portion 242 a ismade thinner, the turn-on voltage will be reduced. Further, the lowerthe concentration of the p-body, the lower the turn-on voltage.

By appropriate selection of the thickness of the thin portion of theoxide layer and reducing the concentration of the p-body, the turn-onvoltage (V_(t)) of the transistor can be less than the turn-on voltage(V_(be)) of the intrinsic diode. For example, the turn-on voltage of thetransistor can be less than 0.6V. Advantageously, by making V_(t) lessthan V_(be) for the low-side transistor of a buck converter, thetransistor can enter third-quadrant conduction during deadtime, causingcurrent to travel through the transistor instead of the body diode.

Lowering V_(t) such that the current goes through the transistor insteadof the body diode can eliminate the reverse recovery time, therebygreatly enhancing the efficiency of the buck converter. Additionally,power is saved because there is no need to discharge the minoritycarriers that would otherwise form the reverse recovery charge on thebody diode during reverse conduction. If the turn-on voltage of thetransistor is too low, however, the ringing caused by switching betweenthe high-side transistor and low-side transistor can unintentionallyactivate the gate. Therefore, the thickness of the thin portion and theconcentration of the p-body can be balanced such that the turn-onvoltage of the transistor is between 0.4V and 0.5V.

FIG. 6 shows an exemplary graph of net doping vs. distance into thep-body from a top surface of the p-body 422 a (along the line 502 fromFIG. 5). In the exemplary embodiments shown in FIG. 6, a thin oxidelayer of 35 Å requires a surface doping concentration of approximately2×10¹⁷ atoms/cm². A thin oxide layer of 70 Å requires a surface dopingconcentration of approximately 5×10¹⁶. Moreover, a thin oxide layer of125 Å requires a surface doping concentration of less than 1×10¹⁶.Therefore, the thinner the thin oxide layer is, the greater the maximumdoping concentration can be to achieve a V_(t) that is less than V_(be).

As shown in the exemplary embodiments of FIGS. 7A-8B, a device having athin oxide, e.g. less than 100 Å, such as 35 Å (FIGS. 7A-7B) or 70 Å(FIGS. 8A-8B), and the proper p-body concentration, a current throughthe transistor (IS) can be much greater than the current through thediode (IB). Moreover, provided that the applied voltage is betweenapproximately 0.4V and 0.8V, no current will go through the diode.However, referring to FIGS. 9A and 9B, if the thin oxide thickness risesto above 100 Å, such as 125 Å, then a much smaller voltage window, e.g.between 0.65 and 0.8V is available in which no current goes through thediode. Moreover, more doping steps are required to get the necessaryp-body concentration.

Varying the Vt of the low-side transistor requires additionalsemiconductor processing steps. To achieve a Vt of approximately 0.4 V,it is helpful to fashion the transistor so that the oxide beneath thegate is thinner where it contacts the source of the transistor thanwhere it contacts the drain of the transistor because the thinner theoxide, the lower the Vt for a given surface doping. This thin oxide isnot suitable for the drain side of the gate, however, because it wouldcompromise the breakdown voltage (BVDSS) of the transistor. Accordingly,the must be shaped as a “step,” with the drain side being thicker thanthe source side.

Referring to FIG. 10A, the process of making the transistor begins byforming an oxide layer 424 on a silicon layer 110.

Referring to FIG. 10B, the oxide layer is patterned to define the gateoxide regions. In addition, a step 902 is formed in the oxide layer 424,creating a thin oxide portion 424 a and a thick oxide portion 424 b. Thethin oxide portion 424 a can be on the source side of the gate, whereasthe thick oxide portion 424 b can be on the drain side of the gate. Thestep 902 could be formed before or after the oxide layer is patterned.The thin oxide portion 424 a can also be deposited simultaneously on anyCMOS devices on the substrate, e.g., using a single mask.

In some implementation, the step 902 in the oxide can be formed bygrowing a thin oxide layer, masking the substrate (including the thinportion 424 a) except for where the thick portion is desired, anddepositing, e.g., using chemical vapor deposition, the remaining oxidein the unmasked area to form the thick portion 424 b. In otherimplementations, the step 902 can be formed by growing a thick oxidelayer, masking the substrate (including the thick portion 424 b) exceptwhere the thin portion is desired, etching the exposed portion of theoxide layer down to the silicon layer, and growing the thin oxide layer424 a in the region that was etched away, e.g., using the same mask thatwas used in the etching step. In either process, the mask can then beremoved.

Referring to FIG. 10C, the gate conductor 102, e.g. a polysilicon layer,is applied over the oxide layer 424. The gate conductor, e.g.polysilicon layer, can be approximately 0.2-0.5 μm thick depending onthe base process technology node. The gate conductor 102 can also bedeposited simultaneously on any CMOS devices on the substrate, e.g.,using a single mask.

Referring to FIG. 10D, photoresist 104 is deposited, e.g., by spincoating, over the gate conductor 102, and patterned to expose at leastthe source side of the transistor. The photoresist can have a thicknessof greater than 0.5 μm. Optionally, some portions of the gate conductor102 on the source side 120 can also be exposed.

Referring to FIG. 10E, the exposed portion of the gate conductor 102 onthe source side 120 of the transistor is then removed by etching usingthe photoresist 104 as a mask, e.g., using dry plasma ecthing. Thephotoresist 104 can thus act as a mask during the etch.

Referring to FIG. 10F, the p-body 422 is implanted using the combinedphotoresist 104 and remaining gate conductor 102 as a mask. The p-body422 is implanted by bombarding the surface of the oxide 424 a with theimplant atoms. The implant can be performed at an angle to the mainsurface of the oxide 424 a (shown by arrow 108). Because both the gateconductor 102 and the photoresist 104 are used as a mask, the resultingp-body 422 can be self-aligned to the gate, particularly to the sourceside of the gate conductor.

Referring to FIG. 10G, the photoresist layer 104 is then stripped fromthe surface.

Referring to FIG. 10H, a new layer of photoresist 124 is applied, e.g.,by spin coating, over the exposed surfaces, including the exposedportion of the oxide layer 424 a on the source side 120 following theetch of the gate conductor 102 on the source side 120. The photoresistlayer 104 is patterned to expose at least the drain side of thetransistor. Optionally, some portions of the gate conductor 102 on thedrain side 122 can be exposed.

Referring to FIG. 10I, the exposed portion of the gate conductor on thedrain side 122 is then removed by etching, e.g., dry plasma etching,using the photoresist 124 as a mask. The photoresist 124 also serves toprotect the implanted p-body on the source side 120 during the etchingprocess.

Referring to FIG. 10J, the NDD 415 can be implanted using the combinedphotoresist 124 and remaining gate conductor 102 as a mask. Because boththe gate conductor 102 and the photoresist 124 are used as a mask, theNDD 415 can be self-aligned to the gate, particularly to the drain sideof the gate conductor.

Referring to FIG. 10K, the photoresist 124 is then removed.

The p+ and n+ regions 414, 418, 420, can be implanted by conventionalimplanting processes, e.g., after the P-body 422 and NDD 415 have beenimplanted.

Although the fabrication of only one gate is illustrated in FIGS. 10a-10 g, multiple gates, or the gate of a distributed transistor, can befabricated simultaneously.

By including a photoresist layer over the polygate during the p-bodyimplantation, a self-aligned p-body can be formed. Moreover, using thephotoresist protects the remainder of the substrate from impuritiescaused during the implantation process. Further, by having two separateetching steps, before and after the implantation process, a singleself-aligned p-body can be formed in the source side of the substrate.

Particular embodiments have been described. Other embodiments are withinthe scope of the following claims.

1. A method of making a transistor, comprising: applying a photoresistover a gate, the gate including an oxide layer formed over a substrateand a conductive material formed over the oxide layer; using thephotoresist as a mask, etching the gate to remove a portion of theconductive material; and using the photoresist and conductive materialas a mask, implanting an impurity region into the substrate such thatthe impurity region is self-aligned to the gate.
 2. The method of claim1, further comprising forming the oxide layer such that a first side ofthe oxide layer is thinner than a second side of the oxide layer.
 3. Themethod of claim 2, wherein the first side has a thickness of less thanabout 100 Å, and the second side has a thickness that it at least fivetimes the thickness of the first side.
 4. The method of claim 1, whereinthe photoresist is less than about 0.5 μm thick.
 5. The method of claim1, wherein implanting an impurity region includes bombarding thesubstrate with atoms at an angle that is less than 90° from a mainsurface of the substrate.
 6. The method of claim 1, wherein implantingan impurity regions continues until a doping concentration of thesubstrate is between about 1×10¹³ atoms/cm² and 5×10¹⁸ atoms/cm².
 7. Amethod of making a transistor, comprising: etching a first side of agate, the gate including an oxide layer formed over a substrate and aconductive material formed over the oxide layer, the etching removing afirst portion of the conductive material; implanting an impurity regioninto the substrate such that the impurity region is self-aligned; andetching a second side of the gate to remove a second portion of theconductive material.
 8. The method of claim 7, further comprisingforming the oxide layer such that a first side of the oxide layer isthinner than a second side of the oxide layer.
 9. The method of claim 8,wherein the first side has a thickness of less than about 100 Å, and thesecond side has a thickness that it at least five times the thickness ofthe first side.
 10. The method of claim 7, wherein implanting animpurity region includes bombarding the substrate with atoms at an thatis less than 90° from a main surface of the substrate.
 11. The method ofclaim 7, wherein implanting an impurity regions continues until a dopingconcentration of the substrate is between about 1×10¹³ atoms/cm² and5×10¹⁸ atoms/cm².
 12. A method of making a transistor, comprising:applying a first photoresist over a gate, the gate including an oxidelayer formed over a substrate and a conductive material formed over theoxide layer; using the first photoresist as a mask, etching a first sideof the gate to remove a first portion of the conductive material;implanting an impurity region into the substrate such that the impurityregion is self-aligned; applying a second photoresist over the gate and;and using the second photoresist as a mask, etching a second side of thegate to remove a second portion of the conductive material.
 13. A methodof making a integrated circuit, comprising: forming a plurality of LDMOStransistors on a substrate, each LDMOS transistor including a gate oxidelayer comprising a first side closer to a source side of the LDMOStransistor and a second side closer to the drain side of the LDMOStransistor, the first side having a thickness of less than about 100 Å,and the second side having a thickness equal to or greater than 125 Å;forming a plurality of CMOS transistor on the substrate, wherein eachCMOS transistor includes a gate oxide layer, and wherein forming thegate oxide layer of the CMOS transistor occurs simultaneously withforming the first side of the gate oxide layer of the LDMOS transistor.14. The method of claim 13, wherein the gate of oxide layer of the CMOStransistor is formed with the same thickness as the first side of thegate oxide layer of the LDMOS transistor.
 15. The method of claim 13,wherein the thickness of the second side is at least five times thethickness of the first side.
 16. The method of claim 13, wherein thethickness of the first side is about 70 Å or less.
 17. The method ofclaim 16, wherein the thickness of the first side is about 35 Å or less.18. The method of claim 13, wherein forming the plurality of LDMOStransistors includes depositing an LDMOS gate conductor and forming theplurality of CMOS transistor includes depositing a CMOS gate conductor,and the LDMOS gate conductor and CMOS gate conductor are depositedsimulataneously.
 19. The method of claim 13, wherein LDMOS gateconductor and CMOS gate conductor comprise polysilicon.
 20. The methodof claim 13, wherein the gate oxide layer of the CMOS gate is asubstantially uniform thickness.